In a drilling rig for oil, gas or coal bed methane, it is conventional to mount a rotating flow control diverter (RFCD) at the top of a blowout preventer (BOP) stack beneath the drilling floor of the drilling rig. The rotating flow control diverter prevents the unintentional escape of well fluids (such as drilling mud, produced fluids and gases, and surface-injected air or gas into a recovery line) by containing and diverting them from the wellbore annulus away from the rig floor. At the same time, the rotating flow control diverter allows a drill string to be passed into and out of the wellbore, and rotated within the wellbore. A typical rotating flow control diverter comprises a stationary housing adapted for incorporation into a wellhead and a rotating tubular shaft with a rubber sealing element to establish a seal with a tubular such as tubing or drill pipe that is passed through the tubular shaft. The tubular shaft is rotatably and axially supported in the stationary housing by an internal bearing assembly comprising bearings and a seal assembly for isolating the bearings from well fluids. The bearing assembly typically comprises an inner race fixed to the outer surface of the tubular shaft, and an outer race fixed to the inner surface of the housing. In use, the tubular, the tubular shaft and sealing element rotate together within the housing.
It is known in the industry to mount a second or upper stripper element (commonly known in the industry as the “dual stripper”) on top of a rotating flow control diverter to enhance the control of wellbore fluids and gases and to provide an additional safeguard against the unintentional escape of wellbore fluids and gases, in the event that the main stripper element of the rotating flow control diverter fails. Prior art rotating flow control diverters with dual strippers typically have both stripper elements attached to a common tubular shaft running through the rotating flow control diverter. Accordingly, if the bearing assembly supporting that tubular shaft fails, then both of the stripper elements may cease to rotate freely within the housing. If the drill string continues to rotate, then the friction between the drill string and the stripper elements will wear and damage the stripper elements, and possibly cause them to fail. Furthermore, prior art rotating fluid control diverters with dual strippers are typically ill equipped to deal with the treatment and removal of gaseous and liquid substances trapped between the two stripper elements.
There is need in the art for an improved rotating control flow diverter with dual strippers that is relatively simple and robust.